Why/how does bolt size effect impact wrench torque?

[marlinspike]

I've read about how the rated torques are measured at a particular bolt size. How/why does bolt size effect the torque the tool is capable of?

 

[AceofSpad3s]

Probably for the sake of constancy.

 

[Deadsquiggles]

Cause bolt size determines How much torque the bolt can handle. It might be rated at 1100 ft/lbs on a 5/8ths bolt. But 1100 ft/lbs might snap a 1/2 inch bolt, so you can really get a rating from that. That's be my guess.

 

[larry_g]

I've read about how the rated torques are measured at a particular bolt size. How/why does bolt size effect the torque the tool is capable of?

Can you post up a link to your reading site? I'm not sure that I understand whether your are talking about a fastener or a tool? If a tool which specific one?

lg
no neat sig line

 

[rancherbill]

I've read about how the rated torques are measured at a particular bolt size. How/why does bolt size effect the torque the tool is capable of?

The tool can do what it can do. The values you are seeing are how you should operate the tool.

The max torque values for a fastener relate to the maximum torque before the fastener will fail. A fastener will fail by breaking or stripping the threads.

Intuitively you know a small fastener will break or strip much easier than a big one.

 

[autoxvaliant]

Kinda unrelated just made me think off it.

Sometimes when I am impacting together suspension on class 8 truck with my MG725 with the Pennsylvania lock-tight (rust) I laugh and wonder about the next poor sap that has to take it apart.

 

[Provincial]

Kinda unrelated just made me think off it.

Sometimes when I am impacting together suspension on class 8 truck with my MG725 with the Pennsylvania lock-tight (rust) I laugh and wonder about the next poor sap that has to take it apart.

Hot Wrench!

 

[chruler]

Two other factors are the diameter and type of bolt head.

A socket head cap screw (allen key) is going to have a different torque spec than the same size bolt with a full-size hex head.

The wider the the torquing area, as in a full-sized hex head, the greater the potential for torque.

Now let's talk about thread pitch.................

 

[DodgeMech]

The bigger the bolt, the more force it takes to get to a certain torque...and pitch plays a role as well, of course...most 1/2 inch impacts are rated on 5/8 bolts

 

[logical]

I have a Mechanical Engineering degree and decades of experience with mechanical fasteners. I don't understand the original question or any of the answers.

OP... start us over with more specifics.

 

[FMC1959]

I've read about how the rated torques are measured at a particular bolt size. How/why does bolt size effect the torque the tool is capable of?

A torque wrench will have a max it can do. What you may have misinterpreted is for a given fastener, the max the wrench should be set at.

 

[GSteg]

Need to clarify the question better.

 

[sac02]

I have a Mechanical Engineering degree and decades of experience with mechanical fasteners. I don't understand the original question or any of the answers.

OP... start us over with more specifics.

Similar professional situation here, though not that much experience specifically with fasteners. I agree that the question is confusing, and the answers are nonsensical in some cases.

The short answer is that bolt size does not affect the capability of the tool to generate torque. This would be like asking "Why does bolt size affect how strong my arm is when hand torquing a fastener?" Obviously, size, pitch, head style, coating/lube, etc plays a factor in how much torque a fastener can sustain and what the clamping force will be for a given torque, but it will not influence how much torque the tool can generate. Things like air pressure and CFM will influence the tool's capability, however.

You see impacts rated any number of ways (some may refer to a bolt size, some may not, it doesn't matter) because there is NO standard for rating them. That makes cross-manufacturer comparisons highly meaningless, and makes it easy for a manufacturer to design a scenario where they can justify publishing inflated numbers that users will never be able to achieve in real-world scenarios.

 

[franzdom]

This thread makes me sad

 

[WVBrady]

I can't resist taking a shot at it. Perhaps the bolt acts a little like a torque stick. A torque stick will only transmit a certain, calibrated torque from an impact wrench, because the torsional spring characteristic of it absorbs part of the pulsating torque. If you put a constant torque on it, it would not do that. I would think that this would be a very small effect, but theoretically it could cause some difference.

 

[djb2]

Torque at the drive fixture is the same, no matter the type of fastener.

The only context I've seen a fastener diameter specified is (paraphrasing) "this torque wrench is good for up to about 5/8 inch bolts". That's not a precise rating, or even a good one, but for someone that doesn't have a knowledge of fastener torque ratings it is more useful. They almost certainly know "hey, my largest bolt is 1/2 inch. This should work for me."

You could even argue that it's a better metric than the torque rating. The torque rating is usually grossly exaggerated, especially with battery tools. A 650 ftlb rating might mean "using a specially built battery pack with extra large low-internal-resistance cells, fully charged, after impacting for five minutes it takes 650 ftlbs to remove the fastener".

 

[jd_1138]

Smaller fasteners have smaller amounts of metal, thus they can handle less torque before breaking or stripping out. It's like a carriage bolt; the larger the carriage bolt, the more weight it can support or hold up.

 

[zkling]

You torque a 1/4"NC bolt to 75ft lbs, then you torque a 1/2"NC bolt to 75ft lbs. Which has the higher torque value? What value is different that is dependent on torque?

 

[mikehaugen]

I can't resist taking a shot at it. Perhaps the bolt acts a little like a torque stick. A torque stick will only transmit a certain, calibrated torque from an impact wrench, because the torsional spring characteristic of it absorbs part of the pulsating torque. If you put a constant torque on it, it would not do that. I would think that this would be a very small effect, but theoretically it could cause some difference.

This was kind of my initial thinking. Impacts tighten things differently than a wrench. I could see using the same impact to torque a 1" bolt and a 3/8" bolt and getting different values, based on friction value and actuall mass of the bolt. I could also see getting different value based on whether you tightened a bolt into a threaded hole or a nut on a stud.

 

[Mooky]

Diameter of the fastener affects the clamping force generated for a given torque:

http://m.machinedesign.com/fastener...threaded-fastener-torque-tension-relationship

This is also the answer to the often asked torque with anti seize question.

 

[dlcwent]

www.engineersedge.com/calculator/torque_calc.htm

may be helpful

 

[larryforce]

I believe this thread is in reference to a forum member who posted a review of the Milwaukee 2673. mode one is supposed to get 100 lbs/ft of torque, but nobody seem to think it was capable of that.
then the poster found out the size and grade of bolt milwuakee used in testing and was able to get 100 lbs/ft on mode 1.

 

[t4runner]

Im not an Mechanical Engineer but I did stay at a Holiday in once......

 

[JCByrd24]

I believe this thread is in reference to a forum member who posted a review of the Milwaukee 2673. mode one is supposed to get 100 lbs/ft of torque, but nobody seem to think it was capable of that.
then the poster found out the size and grade of bolt milwuakee used in testing and was able to get 100 lbs/ft on mode 1.

This doesn't make any sense, the bolt grade and size have nothing to do with what the tool is capable of as has been said. Now there are tons of variables in how the member may have been "testing" this that could lead to this conclusion, but the test likely wasn't perfect.

Also, the unit is not lbs/ft (lbs divided by feet), it's ft-lbs or lbs-ft (lbs times feet), quite literally. Torque = force x length.

P.S. I'm a licensed mechanical engineer as well.

 

[Ign]

I believe this thread is in reference to a forum member who posted a review of the Milwaukee 2673. mode one is supposed to get 100 lbs/ft of torque, but nobody seem to think it was capable of that.
then the poster found out the size and grade of bolt milwuakee used in testing and was able to get 100 lbs/ft on mode 1.

That's the 2763 and yeah, mine won't get to 100 ft lbs. I recently checked on an '05 F150 4x4. Mode 1 topped out around 70 ft lbs, but as stated there are a million variables including temperature, wheel mat'l, and angle of countersink seat (although come to think of it stupidity, er I mean hubcentric, shouldn't need much of a countersink). Either way Mode 1 is mostly useless IMO, but this thread isn't about that.

 

[Ign]

FWIW OP I completely understand your question and I always thought of it like axle gearing; you're twisting from the center of the fastener or wheel, as diameter increases you lose effective leverage or power, which is why you gotta re-gear to, say, 4.10 or 4.56 if you step up to 33's or 35's (solely examples for the sake of example of course).

Again for the sake of example take it to the extreme: say your impact wrench can achieve 100 ft lbs on a 1/2" fastener w a 3/4" socket. Now lets say you're using a 12" socket (whatever fastener size that may be), there's no way you can get me to believe that same impact will achieve 100 ft lbs again because the impact wrench is the same. It's simply lost too much mechanical advantage IMO.

None of this speaks to friction which also must be a consideration; even if thread pitches are the same or are proportionate, a larger diameter has more material contact and thus more friction. This, as previously noted, is also why anti-sieze can theoretically distort torque values.

I have no alphabet soup behind my name, and I may very well be wrong, but it makes sense to me and since, in this case, my continued belief(s) as they are, correct or not, have no significant impact on me or the safety of others, will remain the same until someone has personally demonstrated to me otherwise.

 

[larry_g]

Again for the sake of example take it to the extreme: say your impact wrench can achieve 100 ft lbs on a 1/2" fastener w a 3/4" socket. Now lets say you're using a 12" socket (whatever fastener size that may be), there's no way you can get me to believe that same impact will achieve 100 ft lbs again because the impact wrench is the same. It's simply lost too much mechanical advantage IMO.

.

If I follow your thought train and if I replace the impact with a torque wrench then I should have some sort of conversion table for the torque wrench, yes?
We do not make any allowances for the torque reading when we use different size sockets on a torque wrench, so why would we do so when using impact wrench? In your example there is no torque lost due to the diameter of the socket, what is lost in leverage going out to the OD is gained coming back in to the center as torque is measured at the center of rotation, not the outside of the lever. (socket) You are confusing force with torque. Torque is what turns the tire but force is what propels the vehicle forward.

None of this speaks to friction which also must be a consideration; even if thread pitches are the same or are proportionate, a larger diameter has more material contact and thus more friction. This, as previously noted, is also why anti-sieze can theoretically distort torque values.

Now your starting to get into the realm of 'clamping force' Whole 'nother aspect of why we apply a torque to a fastener in the first place. Clamping force is a result of the torque applied to a screw but as you say friction and thread pitch play a bigger part in the equation than torque applied.

I still have no idea what the OP is blathering on about....


lg
no neat sig line

 

[zkling]

Again for the sake of example take it to the extreme: say your impact wrench can achieve 100 ft lbs on a 1/2" fastener w a 3/4" socket. Now lets say you're using a 12" socket (whatever fastener size that may be), there's no way you can get me to believe that same impact will achieve 100 ft lbs again because the impact wrench is the same. It's simply lost too much mechanical advantage IMO.

In extreme cases like that one needs to take transmissibility and the intertia of the socket into account. It will take longer time, but I'd be willing to bet it will get to the same torque value.

 

[sac02]

Ign,

Your understanding of the mechanics of applying torque is slightly flawed. Your understanding of how a longer or shorter lever arm impacts torque is correct: when you apply the same amount of FORCE over a longer lever arm, you get more torque. If you apply 100 pounds force to a 1 foot long breaker bar, you get 100 lb*ft TORQUE. If you apply 100 pounds force to a 6" long breaker bar, you get 50 lb*ft TORQUE. One of your misconceptions is that the socket size matters in this calculation, which it does not - the torque is calculated (generated) based on the distance from the point of application to the center of rotation, it doesn't matter if you have a 10mm socket or a 50mm socket, the center of rotation is the center of the bolt.

The impact gun however, is NOT applying a force to a lever arm. (internally, it has already done exactly that - applied a force to a lever arm, and the output is torque at the anvil) Instead, it is directly applying a moment (torque) to the fastener (or socket, either way you look at it, the principle is the same). Moments are moments, and do not change based on if you are applying them to a 10mm or 50mm fastener. If you apply 100lb*ft of twist at the anvil, you get 100lb*ft of twist at the fastener, regardless of the bolt or socket size. This may not be immediately intuitive, but I assure you it is correct.

I will backpedal on one item though, and it could certainly make a difference. The one way in which fastener size COULD affect torque capability of an impact would be due to the mass of the fastener/socket. This would be easier to imagine again with an extreme example like the difference in mass between a modest size bolt with a 10mm socket turning it, and a very large bolt with a 100mm socket turning it. The 10mm socket weights a few ounces, and the 100mm socket will weight several pounds. If you hand torqued those two sockets, they would react the same way to a steady torque input. BUT the impact gun applies short impulses and the INERTIA of the mass of the system comes into play. With the impact gun, to rotate the faster/socket, you must accelerate the system from still to moving in that short time frame of a single impulse "hit" - that could be difficult to do as masses and inertias get large.

And it is worth noting that this does rely on some assumptions that the torque transmission of the two systems are equal (one socket is not stiffer than the other, etc).

So I could definitely see a scenario where applying torque in impulse style with an impact gun could result in a lower torque reading when you pull torque on a fastener that is significantly more massive than another. I'd have to do some napkin calculations, but I would bet a lot of money that the effect this has on a 1/2" vs 5/8" fastener is incredibly miniscule though. You would need to be talking about significant differences in mass before this effect became significant.

 

[jd_1138]

You torque a 1/4"NC bolt to 75ft lbs, then you torque a 1/2"NC bolt to 75ft lbs. Which has the higher torque value? What value is different that is dependent on torque?

As someone said above, both fasteners are torqued to 75 ft lbs, but the 1/2" one will be held down with more clamping force.

 

[sac02]

Kind of like the age-old riddle that continues to stump 3 year olds (and stupid adults):

"Which weighs more: a pound of lead, or a pound of feathers?"

And yes, there appear to be a lot of misunderstandings about torque vs clamping force here.

 

[GSteg]

People are forgetting that reading torque values is very much dependent on how many can of beers you had prior. My sober reading is okay, but my drunk readings go beyond what my torque wrench can handle.

 

[larry_g]

As someone said above, both fasteners are torqued to 75 ft lbs, but the 1/2" one will be held down with more clamping force.

Assuming the 1/4" fastener did not break or reach yield the finer pitch thread will apply the higher clamping force. So the 1/4" is 20 tpi and the 1/2" is 13tpi then the 20 tpi will exert the larger clamping force. Its a matter of leverage.

lg
no neat sig line